A Store back-end for rdflib to allow for reading and querying HDT documents.
You should have the
Python.hheader available on your system.
For example, for Python 3.6, install thepython3.6-devpackage on Debian/Ubuntu systems.
Installation using pipenv or a virtualenv is strongly advised!
PyPi installation (recommended)# you can install using pip pip install rdflib-hdt # or you can use pipenv pipenv install rdflib-hdt
Requirement: pipenv
git clone https://github.com/Callidon/pyHDT
cd pyHDT/
./install.sh
You can use the rdflib-hdt library in two modes: as an rdflib Graph or as a raw HDT document.
from rdflib import Graph
from rdflib_hdt import HDTStore
from rdflib.namespace import FOAF
# Load an HDT file. Missing indexes are generated automatically
# You can provide the index file by putting it in the same directory as the HDT file.
store = HDTStore("test.hdt")
# Display some metadata about the HDT document itself
print(f"Number of RDF triples: {len(store)}")
print(f"Number of subjects: {store.nb_subjects}")
print(f"Number of predicates: {store.nb_predicates}")
print(f"Number of objects: {store.nb_objects}")
print(f"Number of shared subject-object: {store.nb_shared}")
# Create an RDFlib Graph with the HDT document as a backend
graph = Graph(store=store)
# Fetch all triples that matches { ?s foaf:name ?o }
# Use None to indicates variables
for s, p, o in graph.triples((None, FOAF("name"), None)):
print(triple)
Using the RDFlib API, you can also execute SPARQL queries over an HDT document. If you do so, we recommend that you first call the optimize_sparql function, which optimize the RDFlib SPARQL query engine in the context of HDT documents.
from rdflib import Graph
from rdflib_hdt import HDTStore, optimize_sparql
# Calling this function optimizes the RDFlib SPARQL engine for HDT documents
optimize_sparql()
graph = Graph(store=HDTStore("test.hdt"))
# You can execute SPARQL queries using the regular RDFlib API
qres = graph.query("""
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?friend WHERE {
?a foaf:knows ?b.
?a foaf:name ?name.
?b foaf:name ?friend.
}""")
for row in qres:
print(f"{row.name} knows {row.friend}")
from rdflib_hdt import HDTDocument
from rdflib.namespace import FOAF
# Load an HDT file. Missing indexes are generated automatically.
# You can provide the index file by putting it in the same directory as the HDT file.
document = HDTDocument("test.hdt")
# Display some metadata about the HDT document itself
print(f"Number of RDF triples: {document.total_triples}")
print(f"Number of subjects: {document.nb_subjects}")
print(f"Number of predicates: {document.nb_predicates}")
print(f"Number of objects: {document.nb_objects}")
print(f"Number of shared subject-object: {document.nb_shared}")
# Fetch all triples that matches { ?s foaf:name ?o }
# Use None to indicates variables
triples, cardinality = document.search((None, FOAF("name"), None))
print(f"Cardinality of (?s foaf:name ?o): {cardinality}")
for s, p, o in triples:
print(triple)
# The search also support limit and offset
triples, cardinality = document.search((None, FOAF("name"), None), limit=10, offset=100)
# etc ...
An HDT document also provides support for evaluating joins over a set of triples patterns.
from rdflib_hdt import HDTDocument
from rdflib import Variable
from rdflib.namespace import FOAF, RDF
document = HDTDocument("test.hdt")
# find the names of two entities that know each other
tp_a = (Variable("a"), FOAF("knows"), Variable("b"))
tp_b = (Variable("a"), FOAF("name"), Variable("name"))
tp_c = (Variable("b"), FOAF("name"), Variable("friend"))
query = set([tp_a, tp_b, tp_c])
iterator = document.search_join(query)
print(f"Estimated join cardinality: {len(iterator)}")
# Join results are produced as ResultRow, like in the RDFlib SPARQL API
for row in iterator:
print(f"{row.name} knows {row.friend}") Handling non UTF-8 strings in python
If the HDT document has been encoded with a non UTF-8 encoding the previous code won't work correctly and will result in a UnicodeDecodeError. More details on how to convert string to str from C++ to Python here
To handle this, we doubled the API of the HDT document by adding:
search_triples_bytes(...) return an iterator of triples as (py::bytes, py::bytes, py::bytes)search_join_bytes(...) return an iterator of sets of solutions mapping as py::set(py::bytes, py::bytes)convert_tripleid_bytes(...) return a triple as: (py::bytes, py::bytes, py::bytes)convert_id_bytes(...) return a py::bytesParameters and documentation are the same as the standard version
from rdflib_hdt import HDTDocument
document = HDTDocument("test.hdt")
it = document.search_triple_bytes("", "", "")
for s, p, o in it:
print(s, p, o) # print b'...', b'...', b'...'
# now decode it, or handle any error
try:
s, p, o = s.decode('UTF-8'), p.decode('UTF-8'), o.decode('UTF-8')
except UnicodeDecodeError as err:
# try another other codecs, ignore error, etc
pass
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